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2.3.2Topic 2 · Genes and Health

Translation — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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2.3.2 Translation — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the stages of translation at the ribosome and explain how mRNA, tRNA and rRNA work together to build a polypeptide.Translation is the second stage of protein synthesis.It is the process in which the sequenc…

  • Transfer RNA (tRNA)
  • Amino acids
  • Initiation
  • Elongation
  • peptide bond
  • Termination

Section 1 of 9Where and why translation happens

Translation is the second stage of protein synthesis.

It is the process in which the sequence of codons in messenger RNA (mRNA) is decoded to produce a specific sequence of amino acids joined together as a polypeptide.
Ribosomes
It occurs on ribosomes in the cytoplasm, either free or attached to the rough endoplasmic reticulum.
Proteins made on the rough ER are usually destined for secretion or for membranes.
Translation follows transcription, in which a gene was copied into mRNA in the nucleus.

Section 2 of 9Components needed

Components needed
mRNA
mRNA carrying a sequence of codons, each a triplet of bases.
Ribosomes
Ribosomes made of a small and a large subunit, each made of ribosomal RNA (rRNA) and protein.
  • The large subunit has two binding sites that hold tRNA molecules next to each other.
Transfer RNA (tRNA)
Transfer RNA (tRNA) molecules, each with an anticodon at one end and a specific amino acid attached at the other.
Amino acids
Amino acids available in the cytoplasm.
ATP
ATP for attaching amino acids to tRNA and for the movement of the ribosome.
Enzymes
Enzymes including aminoacyl-tRNA synthetases that load each tRNA with its correct amino acid.

Section 3 of 9The stages step by step

1
Initiation.
  • The small ribosomal subunit binds to the mRNA near its 5' end and slides along until it finds the start codon, AUG.
  • A tRNA with the anticodon UAC, carrying methionine, pairs with this codon by complementary base pairing.
  • The large subunit then joins.
2
Elongation.
A second tRNA, with an anticodon complementary to the next codon, enters the adjacent binding site.
  • The amino acids on the two tRNAs are brought close together and a peptide bond forms between them in a condensation reaction, catalysed by the ribosome using energy from ATP.
  • The first tRNA is released and returns to the cytoplasm to pick up another amino acid.
  • The ribosome moves along the mRNA by one codon (translocation), so a new codon is exposed in the free site and the cycle repeats.
  • The polypeptide chain lengthens by one amino acid each cycle.
3
Termination.
  • When the ribosome reaches a stop codon, no tRNA has a matching anticodon.
    • (UAA, UAG or UGA)
  • Release factors bind, the polypeptide is released and the ribosome subunits separate from the mRNA.

Section 4 of 9Polysomes and efficiency

Several ribosomes can attach to a single mRNA molecule at once, each at a different stage of translation.

Polysome
This structure is called a polysome (or polyribosome).
It allows many copies of the same polypeptide to be made from one mRNA in a short time.
After translation, the polypeptide may be folded, modified in the Golgi apparatus (for example by adding carbohydrate groups) and transported.
Individual mRNA molecules are eventually broken down, so a cell can change its protein production quickly in response to need.

Section 5 of 9Template and complementary sequences

LevelExample sequence
DNA template strandTAC CGA TTT ACT
mRNA codonsAUG GCU AAA UGA
tRNA anticodonsUAC CGA UUU ACU
Amino acidsMet - Ala - Lys - stop

Note that the final codon is a stop codon, so no amino acid is added. The anticodons are the same as the DNA template with U in place of T.

Section 6 of 9Errors, regulation and links

A change in the DNA base sequence (a mutation) alters the mRNA codons and may change the amino acid sequence, and therefore the shape and function of the protein, as in sickle cell anaemia.

Errors, regulation and links
Antibiotics such as tetracycline and streptomycin work by blocking bacterial ribosomes,
  • which are 70S and differ from the 80S ribosomes of eukaryotes, stopping bacterial protein synthesis without greatly affecting human cells.
The sequence of events here links to the genetic code, to protein structure and to enzyme function in the rest of this topic.

Putting the numbers together helps in calculation questions.

1
A polypeptide of 150 amino acids needs an mRNA coding region of 150 codons,
  • which is 450 bases, plus a stop codon,
  • so 453 bases in the coding sequence.
2
The mRNA is therefore always longer than the polypeptide suggests, and the gene is longer still in eukaryotes because of introns.
3
A question may give the number of amino acids and ask how many peptide bonds formed (149) or how many tRNA molecules were used (150, though each can be reused).
4
Also remember that energy is used at several points: ATP loads each amino acid onto its tRNA, and GTP provides energy for translocation.
5
Linking translation back to transcription earns marks in longer answers: transcription produces the template, and translation reads it.

Section 7 of 9Exam-style questions

  1. Describe how a peptide bond is formed during translation. [3]3 marks

  2. A section of mRNA reads AUGGUCCAUUAG. Give the anticodon sequence of the tRNA that brings the second amino acid and name the process that stops translation. [3]3 marks

  3. Explain the advantage of polysomes. [2]2 marks

Section 8 of 9Key terms

TermMeaning
TranslationSynthesis of a polypeptide at a ribosome using the sequence of codons on mRNA.
CodonA triplet of bases on mRNA specifying an amino acid or stop.
AnticodonA triplet of bases on tRNA complementary to a codon.
Start codonAUG, which codes for methionine and begins translation.
Stop codonA codon (UAA, UAG, UGA) that ends translation.
Peptide bondCovalent bond between amino acids formed by condensation.
PolysomeSeveral ribosomes translating a single mRNA at the same time.

Section 9 of 9Exam technique

1
Confusing translation with transcription. Transcription makes mRNA in the nucleus; translation makes a polypeptide at the ribosome.
2
Writing that tRNA codons pair with mRNA anticodons. mRNA has codons and tRNA has anticodons.
3
Forgetting that ATP is needed to load amino acids onto tRNA.
4
Saying that the ribosome moves the mRNA. The ribosome moves along the mRNA.

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